Alkaline Paper Advocate

Volume 8, Number 4
Dec 1995


Contributions of the Central Research Laboratory to the Field of Conservation and Restoration. Edited by Hugo Verschoor and Jaap Mosk. 1994. ISBN 90-72905-37-7. Dfl 50 [$32.50] from Central Research Laboratory, Gabriël Metsustraat 8, 1071 EA Amsterdam, Netherlands, (31-20) 673 51 62, fax (31-20) 675 16 61. [dfl=guilders] 124 pages, in English.

Reviewed by Ellen McCrady

This book reports work done in 1992 and 1993. Two of the nine reports have been chosen, because of their relevance to paper research.

"Research into the Cause of Browning of Paper Mounted in Mats," by Judith H. Hofenk de Graaff (p. 21-42). The work on this project was done by and in cooperation with Maarten van Bommel (CRL) and Dr. Henk Porck and Sophia Pauk of the Royal Library, among others.

The phenomenon that this research effort focussed on is not very conspicuous, which may explain why it was not investigated earlier. It was first noticed by conservators at the Municipal Archives of Amsterdam in 1989, but was found to be common in other institutions as well. The explanation is based on inhouse and published research on tidelines, some kinds of foxing, and the aging effect of fluctuating relative humidity, which are not three separate phenomena, but manifestations of the damage done to paper when water evaporates from it.

The problem was that prints and drawings--all of them, not just the top one--turned brown within the window opening, even if the matboard was archival and even if the mats were stored with other mats inside a box, in a climate-controlled museum or archives. This cannot be blamed on poor matboard, or light, or any of the usual causes of browning. It cannot even be blamed on fluctuations of relative humidity outside the box. (This has an effect, as the researchers discovered, but a very small one.)

It turned out to be an effect of temperature fluctuation, which drives an RH fluctuation inside the box in a most counter-intuitive way. Because the enclosure is heavily humidity-buffered (that is, the matboard can absorb a lot of moisture from the air and give it off again, like silica gel does), a rising temperature drives the moisture out of the board just as it drives moisture out of silica gel when it is reconditioned in the oven. This raises the RH, making temperature and RH vary together. When the temperature falls, the RH falls, the paper in the opening loses moisture, and turns brown as it oxidizes over its surface. The browning caused by evaporation of water from the surface becomes more distinct with aging, because the oxidized area ages faster. Some foxing spots started out as moistened areas.

The research is thorough and well-designed, and the report is clearly written and stimulating to read. The study seems to support Chandru Shahani's research at the Library of Congress on the effect of fluctuating RH, and his comments on the browning, decrease in pH and weakening in the margins of books as well: these aging effects may be due to temperature or RH fluctuations as much as to pollutant gases.

Can a storage room in a library be regarded as an enclosure, to which this study's results apply? How will the results affect recommendations for environmental control and storage methods?

Why did the prints, and not the mats, turn brown? Did it have anything to do with the surface-to-volume ratio?

One wonders whether other materials besides cellulose react this way upon drying, and what other conditions may cause or prevent the phenomenon. Until the use of chlorine became general, part of the process of bleaching linen fabrics was to spread them out on grass in the sun and sprinkle them several times daily with water, which shows that browning does not always occur upon drying.

Another apparent preventive measure, or at least a practical way to suppress tidelines, was demonstrated by Johannes Hyltoft in the early 1980s in the Smithsonian Institution Book Conservation Lab. After part of a paper was wet, he held it up to a fan to make it dry rapidly. No visible tideline formed. Was this because drying speeded up the drying process and there was not enough time for the oxidation to take place?

The report has graphs, tables, formulas, and technical passages describing the test methods used, previous research, test results, and underlying chemistry.

"Investigation of the Long-Term Effects of Ethylene Oxide and Gamma Rays on the Aging of Paper," by Judith H. Hofenk de Graaff and Wilma G. Th. Roelofs (p. 53-64). Despite the drawbacks of ethylene oxide (EtO) and gamma ray treatment for mold-infested materials, they are still widely regarded as the safest and most effective options for paper materials. EtO sterilization has been banned for use with items of cultural value in the Netherlands because of an explosion at a private company that did work for cultural institutions, and the suspicion that EtO was carcenogenic. Now the remaining option for records and books is gamma radiation.

The effect of these two methods on paper was compared. Seven tests of chemical and physical properties were done on five papers: Whatman No. 5, a normal alkaline buffered photocopy paper, a document paper meeting permanence standards, newsprint and a late-19th century hard-sized cotton and linen paper. These tests, performed before and after aging for three days by both dry and moist aging methods, were folding endurance, internal tearing resistance, stiffness, bursting strength, pH, viscosity and copper number.

They found that ethylene oxide treatment did not affect the aging of paper. Gamma irradiation, however, had a strong negative effect. Treatment for one hour using 10 kGy accelerated the aging process 50-100%, depending on the type of paper. The effect was strongest in the first part of the aging process. Subsequent treatments, of course, would add to the effect. Oddly, the better quality papers deteriorated more quickly, especially by dry aging, showing that oxidation was more important for them. The reverse was true of the newsprint and the 19th century paper, showing that the dominant mechanism for them was acid hydrolysis.

The copper number of the irradiated samples, a measure of stability, was determined again a year later, and was found to have doubled in the interim--a delayed effect, indicating an increase in the number of reducible groups such as carbonyl groups. Previous research using a similar dosage level, done in Czechoslovakia and reported in 1984 in Restaurator, had found that gamma radiation had no effect on the material, but did not include follow-up testing.

Viscosity is an index of degree of polymerization (DP). A decrease of DP from 3300 to 880 was said by G.O. Phillips in 1985 to have no noticeable effect on mechanical properties. However, oxidation and chain cleavages together cause accelerated aging which can be observed in the mechanical properties at a later stage. (3B1)

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